1. Field of the Invention
This invention relates generally to magnetic recording systems, and more particularly to a write head with a spin-torque oscillator (STO) that provides microwave-assisted magnetic recording (MAMR).
2. Description of the Related Art
Perpendicular magnetic recording (PMR) in magnetic recording hard disk drives, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the magnetic recording layer of the disk, allows for ultra-high recording density, i.e., the areal density of the recorded bits on the disk. However, an increase in recording density requires a corresponding reduction in the size of the magnetic grains in the magnetic recording layer to achieve sufficient medium signal-to-noise ratio. As the size of the magnetic grains is reduced, the magnetocrystalline anisotropy of the magnetic grains must be increased to maintain adequate thermal stability. Simultaneously, the magnetic write field from the write head has to exceed the coercivity of the magnetic recording layer to achieve saturation digital recording, resulting in a conflicted limitation on the anisotropy of the magnetic grains.
A PMR system with high-frequency assisted writing using a spin-torque oscillator (STO) has been proposed. This type of recording, also called microwave-assisted magnetic recording (MAMR), applies a high frequency oscillatory auxiliary magnetic field from the STO to the magnetic grains of the recording layer. The auxiliary field may have a frequency close to the resonance frequency of the magnetic grains in the recording layer to facilitate the switching of the magnetization of the grains at lower write fields from the conventional write head than would otherwise be possible without assisted recording. Conversely, MAMR may be used to increase the coercivity of the magnetic recording layer above that which could be written to by a conventional write head alone. The increase in coercivity afforded by MAMR allows for a reduction in the size of the magnetic grains and thus a corresponding increase in recording density. MAMR systems are described in U.S. Pat. No. 6,785,092 B2; US 2008/0137224 A1; and by J. G. Zhu et al., “Microwave Assisted Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008, pp. 125-131.
In proposed MAMR write heads, the STO is located on the head carrier or slider between the write pole and the trailing magnetic shield. The STO is a multilayer film stack made up of two or more magnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One of the magnetic layers, the field generating layer (FGL), is designed to have its magnetization orientation oscillate in the presence of a direct electrical current perpendicular to the film planes of the film stack. In the presence of current above a critical current density, another magnetic layer, referred to as the reference layer and whose magnetization orientation preferably does not oscillate, acts as the “spin-polarizer” to produce a spin-polarized current at the FGL. This destabilizes the static equilibrium of the FGL's magnetization orientation, causing it to undergo sustained oscillation at frequencies useful for MAMR applications.
The STO Cu spacer layer is susceptible to corrosion, which can occur when the air-bearing surface (ABS) of the slider is exposed to the atmosphere and the Cu oxidizes. The electrical contacts for the STO are above and below the STO because the current is directed perpendicularly through the planes of the STO layers. Thus a method for detecting corrosion by measuring the electrical resistance of the STO using the existing electrical contacts is not reliable because the spacer layer is so thin that the resistance value and change in resistance is too small. Also, the small size and location of the STO in the MAMR head makes it difficult to reliably detect corrosion by imaging the STO using scanning electron microscopy (SEM).
Because it is important during high volume production of sliders with MAMR heads to be able to identify those that have unacceptable levels of STO corrosion, a reliable STO corrosion detection system and method is essential.